Networking with Wi-Fi like ConnectivitySlide 2What are White Spaces?Slide 4The Promise of White SpacesGoal: Deploy Infrastructure WirelessSlide 7White Spaces Spectrum AvailabilityWhite Spaces Spectrum AvailabilityWhite Spaces Spectrum AvailabilityWhiteFi SystemKNOWS White Spaces PlatformWhiteFi System ChallengesDiscovering a Base StationWhitespaces Platform: Adding SIFTSIFT, by exampleBS Discovery: Optimizing with SIFTBS Discovery: Optimizing with SIFTDiscovery: Comparison to BaselineWhiteFi System ChallengesChannel Assignment in Wi-FiSpectrum Assignment in WhiteFiAccounting for Spatial VariationIntuitionMulti Channel Airtime Metric (MCham)WhiteFi Prototype PerformanceConclusions and Future WorkSlide 28Networking with Wi-Fi like ConnectivityVictor Bahl, Ranveer Chandra, Thomas Moscibroda, Microsoft ResearchRohan Murty*, Matt WelshHarvard UniversityWhite Space2Analog TV Digital TVSpain (2010)Japan (2011)Canada (2011)UK (2012)China (2015)….….…..USA (2009)Higher FrequencyWi-Fi (ISM)Broadcast TVdbmFrequency-60-100“White spaces”470 MHz700 MHzWhat are White Spaces?0 MHz7000 MHzTVISM (Wi-Fi)7004702400 51802500 5300 are Unoccupied TV ChannelsWhite Spaces54-90 170-2163Wireless Mic TV Stations in America•50 TV Channels•Each channel is 6 MHz wide•FCC Regulations*•Sense TV stations and Mics •Portable devices on channels 21 - 51Why should we care about White Spaces?4The Promise of White Spaces0 MHz7000 MHzTVISM (Wi-Fi)7004702400 51802500 530054-90 174-2165Wireless Mic More SpectrumLonger RangeUp to 3x of 802.11gat least 3 - 4x of Wi-Fi}Potential ApplicationsRural wireless broadbandCity-wide mesh……..……..Goal: Deploy Infrastructure WirelessAvoid interfering with incumbentsGood throughput for all nodesBase Station (BS)6Why not reuse Wi-Fi based solutions, as is?7White Spaces Spectrum AvailabilityDifferences from ISM(Wi-Fi)8FragmentationVariable channel widths12 3 4 512 3 4 5Each TV Channel is 6 MHz wide Use multiple channels for more bandwidthSpectrum is Fragmented1 2 3 4 5 6 >600.10.20.30.40.50.60.70.8 Urban Suburban Rural# Contiguous ChannelsFraction of Spectrum SegmentsWhite Spaces Spectrum AvailabilityDifferences from ISM(Wi-Fi)9FragmentationVariable channel widths12 3 4 5Location impacts spectrum availability Spectrum exhibits spatial variationCannot assume same channel free everywhere12 3 4 5Spatial VariationTVTowerWhite Spaces Spectrum AvailabilityDifferences from ISM(Wi-Fi)10FragmentationVariable channel widthsIncumbents appear/disappear over time Must reconfigure after disconnectionSpatial VariationCannot assume same channel free everywhere12 3 4 512 3 4 5Temporal VariationSame Channel will not always be freeAny connection can bedisrupted any timeEvaluationDeployment of prototype nodesSimulationsWhiteFi SystemPrototype Hardware PlatformBase Stations and Clients11AlgorithmsDiscovery Spectrum Assignmentand ImplementationHandling DisconnectionsKNOWS White Spaces PlatformNetStackTV/MIC detectionFFTConnection ManagerAtheros Device DriverWindows PCUHF RX DaughterboardFPGAUHF TranslatorWi-Fi CardWhitespace RadioScanner (SDR)12Variable Channel Width Support**Case for Adapting Channel Widths, SIGCOMM 2008Fragmentation Spatial VariationTemporal VariationImpactWhiteFi System Challenges13Spectrum AssignmentDisconnectionDiscoveryDiscovering a Base StationCan we optimize this discovery time?12 3 4 514Discovery Time = (B x W) 12 3 4 5How does the new client discover channels used by the BS? BS and Clients must use same channelsFragmentation Try different center channel and widthsWhitespaces Platform: Adding SIFTNetStackTV/MIC detectionFFTTemporal Analysis(SIFT)Connection ManagerAtheros Device DriverPCUHF RX DaughterboardFPGAUHF TranslatorWi-Fi CardWhitespace RadiosScanner (SDR)SIFT: Signal Interpretation before Fourier Transform15SIFT, by exampleADCSIFTTimeAmplitude1610 MHz5 MHzData ACKSIFSSIFTPattern match in time domainDoes not decode packetsBS Discovery: Optimizing with SIFT12 3 4 512 3 4 5SIFT enables faster discovery algorithmsTimeAmplitude17Matched against 18 MHz packet signature18 MHzBS Discovery: Optimizing with SIFTLinear SIFT (L-SIFT)1812 3 4 512 3 4 56 78Jump SIFT (J-SIFT)Discovery: Comparison to Baseline190 30 60 90 120 150 18000.10.20.30.40.50.60.70.80.91 Linear-SIFT Jump-SIFTWhite Space - Contiguous Width (MHz)D is cove r y T im e R ati o (co m p a re d to b a s e lin e )Baseline =(B x W) L-SIFT = (B/W) J-SIFT = (B/W) 2X reductionFragmentation Spatial VariationTemporal VariationImpactWhiteFi System Challenges20Spectrum AssignmentDisconnectionDiscoveryChannel Assignment in Wi-FiFixed Width Channels21 Optimize which channel to use1 6111 611Spectrum Assignment in WhiteFi12 3 4 522Spatial Variation BS must use channel iff free at clientFragmentation Optimize for both, center channel and width12 3 4 5Spectrum Assignment ProblemGoalMaximize ThroughputIncludeSpectrum at clientsAssignCenter ChannelWidth&Accounting for Spatial Variation2312 3 4 512 3 4 512 3 4 5=12 3 4 512 3 4 512 3 4 512 3 4 5Intuition24BSUse widest possible channelIntuition13 4 52Limited by most busy channelBut Carrier Sense Across All Channels All channels must be freeρBS(2 and 3 are free) = ρBS(2 is free) x ρBS(3 is free)Tradeoff between wider channel widths and opportunity to transmit on each channelMulti Channel Airtime Metric (MCham)25BSρBS(2) Free Air Time on Channel 213 4 52ρBS(2) Contention1ρn(c) = Approx. opportunity node n will get to transmit on channel cρBS(2) = Max (Free Air Time on channel 2, 1/Contention)MChamn (F, W) = ),()(5WFcncMhzWPick (F, W) that maximizes (N * MChamBS + ΣnMChamn)0 10 20 30 40 5000.511.522.533.5 20 Mhz 10 MHz 5 MHzBackground traffic - Packet delay (ms)T h ro u g h p u t ( M b p s )0 5 10 15 20 25 30 35 40 45 5000.511.522.5 20 Mhz 10 MHz 5 MHzBackground traffic - Packet delay (ms)M Cham -value015304560759010512013515016518019521022524000.511.522.533.544.55 WhiteFi OPTSecondsThroughput (Mbps)WhiteFi Prototype Performance2625 31 3226 27 28 29 3033 34 35 36 37 38 39 40Conclusions and Future Work•WhiteFi: White Spaces based wireless network–Go beyond considerations of a single link–Change in spectrum access paradigm•SIFT for quick BS discovery•MCham to assign spectrum•Handling Disconnections•On-going work: Campus wide
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